WO2018015368A1 - Nano-light source emitting polarized light - Google Patents

Abstract

The invention provides a material (100) comprising In_xGa_1-xP, wherein 0<x<1, and wherein In_xGa_1-xP has the Wurtzite crystal structure. The invention also provides a core- shell nanowire, wherein the core-shell nanowire comprises a core of a second material having the Wurtzite crystal structure, and a shell comprising a first material comprising said In_xGa_1-xP. The invention also provides a device comprising a first electrode and a second electrode which are mutually connected via In_xGa_1-xP or the core-shell nanowire. The emission is tunable from orange to far red.

Description

Nano-light source emitting polarized light

FIELD OF THE INVENTION

The invention relates to a novel material that can amongst others be used for luminescence and can be used in a (solid state) lighting device or (solid state) lighting system.

BACKGROUND OF THE INVENTION

Nanowires are known in the art. US2014230720, for instance, describes the growth of GaP and III-V GaP alloys in the wurtzite crystal structure by vapor phase epitaxy (VPE). Such material has a direct band gap and is therefore much more useful for optoelectronic devices than conventional GaP and GaP alloys having the zincblende crystal structure and having an indirect band gap. Core-shell nanowires can be grown via this approach, in which the crystal structure of the core is directly transferred into the shell. For example, wurtzite nanowires of a first III-V composition can be grown as described herein (e.g., using a VLS approach) and then a second composition (different from the first composition) can be grown on the first composition, which has the same wurtzite crystal structure as the first composition. Any growth technique can be used for growth of the second composition, e.g., a vapor-solid (VS) method. The resulting structure will have a core of the first composition and a shell of the second composition with the crystal structure of the first composition.

The paper Nanopillar Lasers Directly Grown on Silicon with Heterostructure

Surface Passivation by Hao Sun et al, ACS Nano, 2014, 8 (7), pp 6833-6839, DOI:

10.1021/nn501481u, describes single-crystalline wurtzite InGaAs/InGaP nanopillars directly grown on a lattice-mismatched silicon substrate are demonstrated. The nanopillar growth is in a core-shell manner and gives a sharp, defect-free heterostructure interface. The InGaP shell provides excellent surface passivation effect for InGaAs nanopillars, as attested by 50- times stronger photo luminescence intensities and 5 -times greater enhancements in the carrier recombination lifetimes, compared to the unpassivated ones. A record value of 16.8% internal quantum efficiency for InGaAs-based nanopillars was attained with a 50-nm-thick InGaP passivation layer. A room-temperature optically pumped laser was achieved from single, as-grown InGaAs nanopillars on silicon with a record- low threshold. Superior material qualities of these InGaP-passivated InGaAs nanopillars indicate the possibility of realizing high-performance optoelectronic devices for photovoltaics, optical communication, semiconductor nanophotonics, and heterogeneous integration of III-V materials on silicon.

The paper Dependence of InGaP nanowire morphology and structure on molecular beam epitaxy growth conditions by A Fakhr et al, Nanotechnology. 2010 Apr 23;21(16): 165601. doi: 10.1088/0957-4484/21/16/165601, describes that InGaP nanowires (NWs) were grown by the Au-assisted method in a gas source molecular beam epitaxy system. The dependence of InGaP composition, morphology and stacking fault density was studied with respect to group III and V impingement rate and size of the Au particle.

Compositional analysis showed that the NWs had an In-rich core and a Ga-rich shell structure. The In incorporation within the NW became limited as the Au seed particle size diminished or the group III and V flux decreased. The NWs had wurtzite (WZ) crystal structure with zinc blende (ZB) segments (stacking faults). The density of the stacking faults decreased as the group III flux decreased and the group V flux increased.

The paper Indium-rich InGaP nanowires formed on InP (lll)a substrates by selective-area metal organic vapor phase epitaxy by Fumiya Ishizaka et al, Japanese Journal of Applied Physics, Volume 52, Number 4S, describes that the growth of indium-rich InGaP nanowires (NWs) on InP (111)A substrates by selective-area metal organic vapor phase epitaxy (SA-MOVPE) was studied. We obtained vertically aligned InGaP NWs by optimizing growth conditions, such as group III supply ratio and V/III ratio. We found that the height, diameter, shape, and composition of InGaP NWs depended significantly on the supply ratios of trimethylgallium (TMGa) and trimethylindium (TMIn). As the supply ratio of TMGa was increased, the lateral growth was drastically enhanced, and the uniformity of NWs deteriorated. Furthermore, the sidewall facets of NWs changed from {211 } to {110} as the supply ratio of TMGa was increased, indicating the possibility of structural transition from wurtzite (WZ) to zinc blende (ZB). We propose a possible growth model for such lateral growth, uniformity, and structural transition. Photoluminescence (PL) measurements revealed that the Ga compositions ranged approximately from 0 to 15%. Our results show that highly uniform InGaP NWs can be grown by controlling the growth conditions.

US2010/295441 describes a light emitting device employing nanowire phosphors. The light emitting device comprises a light emitting diode for emitting light having a first wavelength with a main peak in an ultraviolet, blue or green wavelength range; and nanowire phosphors for converting at least a portion of light having the first wavelength emitted from the light emitting diode into light with a second wavelength longer than the first wavelength. Accordingly, since the nanowire phosphors are employed, it is possible to reduce manufacturing costs of the light emitting device and to reduce light loss due to non- radiative recombination.

SUMMARY OF THE INVENTION

There is a need for alternative solid state lighting devices, especially with tunable emission bands and/or emission in the red and/or having a relative high efficiency. Hence, especially there is need for alternative red LEDs that may have a better efficiency or other advantageous luminescent properties like excitation and/or emission band position and/or emission band width, and/or tuneability of the emission band. Further, there is still a need for obtaining good inorganic luminescent materials that can replace or supplement existing luminescent materials, such as for solid state lighting, for instance because of better efficiency or other advantageous luminescent properties like excitation and/or emission band position and/or emission band width. Further, there is a need for polarized light emitting light sources, such as e.g. for headlights of cars, LCD backlighting, or for projection lighting, in swimming pool lighting, for water way lighting, in indoor (sport) arena lighting, in fashion lighting, in machine vision system lighting, for quality inspection lighting, etc. etc.

Hence, it is an aspect of the invention to provide an alternative (luminescent) material, which preferably further at least partly obviates one or more of above-described drawbacks.

The inventors have found that a specific semiconductor material can have a relative high efficiency dependent upon the structure of the semiconductor material and the composition of the semiconductor material.

Recently, a new degree of freedom in band structure engineering has become available in the form of crystal phase tuning in semiconductor nanowires, allowing the control of the material polytype. Crystal phase engineering thus unlocks new optical and electronic properties while maintaining compatibility with well-established mainstream semiconductor technology. This provides a promising method to surpass conventional limits of commonly used semiconductors.

Various nanowire growth mechanisms enable the formation of unusual crystal phases, such as Wurtzite (WZ) in Ill-Phosphides, and by exploiting 3D epitaxial overgrowth such crystal phase can be transferred from the core into its surrounding shell. This method allows the growth of materials with crystal structures which are not accessible in bulk. Wurtzite nanowires are very promising candidates for solid state lighting, photovoltaics, solar hydrogen conversion and are therefore natural candidates for novel high performance device solutions. Wurtzite Ill-Phosphides enable direct band gap green emitters, opening a clear way to "bridge the green gap".

The inventors achieved the growth of WZ In_xGai__xP alloys through crystal structure transfer in core-shell nanowires and study their optical properties. The inventors demonstrated emission tunability between 590nm (2.1eV) and 760nm (1.63eV), dramatically increasing the Internal Quantum Efficiency (IQE) from 0.01% for WZ GaP to 18% for WZ In_xGai-_xP with x>0.4, beyond the pseudo-direct to direct Tsc - ic crossover point. This crossover also provides an opportunity for thermoelectrics since it leads to substantial band convergence, resulting in a high density of states. The high IQE makes WZ In_xGai__xP suitable for solid-state lighting application in the red and infrared range, while still having a relevant application for solar hydrogen conversion due to its wide and tunable band gap.

The systems described herein may further be optimized to yet even further increase the advantageous properties.

Hence, in a first aspect the invention provides a material comprising In_xGai__xP, wherein 0<x<l, and wherein In_xGai__xP has the Wurtzite crystal structure. InGaP is known to have the zinc blende (ZB) structure. However, with methods as described herein it is possible to obtain InGaP having the Wurtzite (WZ) crystal structure. The Wurtzite structure per se is known in the art and is an archetype crystal structure, having hexagonal symmetry, in contrast to zinc blende (also known as zincblende), which has a cubic symmetry. The hexagonal crystal structure of the In_xGai__xP is clearly visible in e.g. TEM analysis. The novel material WZ In_xGai__xP, i.e. In_xGai__xP having the Wurtzite crystal structure, allows a plethora of possibilities such as luminescence in the orange, red, and far red, or even infrared. Further, the novel material WZ In_xGai__xP can provide polarized light. Further, it is expected that also this material can be used for direct solar light to hydrogen conversion (SLH), analogous to the use of InP in such applications. In_xGai__xP with 0<x<l is herein also indicated as "InGaP".

With respect to the quantum efficiency of the material as luminescent material and in view of the spectral distribution, it appears useful when x is at least 0.2 but not larger than 0.8. Hence, in embodiments 0.2<x<0.8, such as 0.25-0.70 applies. In specific

embodiments, x is equal to or larger than 0.3, such as 0.3<x<0.6. Even more especially, in embodiments 0.35<x<0.6. With such x ranges, red luminescent light can be provided with relative high quantum efficiencies. In specific embodiments, 0.35<x<0.5, such as

0.35<x<0.45. The Wurtzite structure may in embodiments be imposed on the In_xGai__xP by growing the In_xGai__xP on a substrate that has the Wurtzite structure and thereby imposes this structure on the In_xGai__xP material growing on the substrate, or otherwise imposes the In_xGai_ _XP material to be grown as Wurtzite. Therefore, in embodiments the material as defined herein comprising a first material comprising said In_xGai__xP (i.e. wherein 0<x<l , and wherein In_xGai__xP has the Wurtzite crystal structure) and comprising a second material, wherein the second material has the Wurtzite crystal structure, and wherein the first material is attached to the second material. As indicated above, nano wires (NW) may be used as substrate or second material on which the In_xGai__xP may be grown as shell.

Hence, in embodiments the invention also provides a core-shell nanowire, wherein the core-shell nanowire comprises a core of a second material having the Wurtzite crystal structure, and a shell comprising a first material comprising said In_xGai__xP. The material will in general comprise a plurality of nanowires. These nanowires may be substantially the same, though in other embodiments the plurality of nanowires may also include two or more subsets of different types of nanowires. Such differences may be in one or more of length of the nano wires, thickness of the core, composition of the core, thickness of the shell and composition of the shell, or even also a further (protective shell). Hence, the term "shell" may in embodiments also refer to a multi-layer shell, of which at least one layer includes In_xGai__xP as defined herein. In specific embodiments, the core has a diameter (dl) selected from the range of 10-200 nm, such as 20-100, and the shell has a thickness (d2) selected from the range of 5-200 nm, such as 10-50 nm. Especially, the nanowires may have lengths (11) selected from the range of at least 20 nm. As indicated above, such dimensions may substantially be identical for all nanowires, such as with deviations of mean values in the range of 20% or less, like 10% or less, though in other embodiments the variations may be larger and/or two or more subsets with substantially different dimensions may be provided. Differences in dimensions and/or compositions may lead to different electrical and/or optical properties, such as (different) emission band maximum.

The nanowires can be grown on a support, with methods known in the art, and of which embodiments are further elucidated below. Hence, in embodiments the In_xGai__xP can be provided in the form of core-shell nanowires on a support, such as e.g. a GaP support. As shown in the art, the support does not necessarily have or impose the Wurtzite structure. For instance, the support can be GaP, on which GaP WZ nanowires are grown. Hence, in yet a further aspect the invention also provides a system comprising a support and a plurality of core-shell nanowires as defined herein arranged on said support. Especially, the support comprises a single crystalline substrate (a wafer) of e.g. GaP. In general, to obtain vertical nanowires, especially (1 1 1)B is needed. Hence, the support is especially (1 1 1)B oriented. Nano wires can be obtained also on other orientations, such as (1 1 1)A. Hence, in

embodiments the support may be (1 1 1)A oriented. In this case the nanowires will grow at an angle relative to the support (instead of substantially an angle of 90 °). However, other materials than a single crystalline supports than GaP may e.g. be selected from the group consisting of InP and GaAs, and any other material that has a (1 1 1)B orientation (or optionally (1 1 1)A orientation. Probably, even hexagonal materials like GaN and sapphire might be used as support. It is further also referred to US20140230720A1 , which is herein incorporated by reference.

In further specific embodiments, adjacent nanowires have shortest distances (d3) selected from the range of 10-50,000 nm, such as 100-5,000 nm. In specific

embodiments, the nanowires are arranged in a 2D array. The array may especially be substantially regular having a cubic or hexagonal symmetry.

Instead of nanowire cores, also other substrates can be used, though nanowire cores (with the WZ structure) seem the best option.

As indicate above, the term "shell" may in embodiments also refer to a multilayer shell, of which at least one layer includes In_xGai__xP as defined herein. Hence, in embodiments the invention also provides core multi-layer shell nanowires, wherein at least one shell, especially a plurality of shells comprise In_xGai__xP as defined herein. In

embodiments, two or more shells have different types of dopings. In yet further

embodiments, a shell has another type of doping than the core, such as n-type shell and p- type core, or p-type shell and n-type core. Therefore, in specific embodiments, the core-shell nanowires include a radial pn-junction geometry. Such nanowires can be sandwiched between electrodes for creating solid state (lighting) devices (see also below), especially with the core electrically connected with a first electrode and with a shell electrically connected with another electrode.

Hence, the In_xGai__xP provided herein may be used for different applications. Therefore, the invention also provides embodiments of devices. Hence, in a further (general) aspect the invention provides a device comprising at least one or more of (i) the material as defined herein (i.e. In_xGai__xP,wherein 0<x<l , and wherein In_xGai__xP has the Wurtzite crystal structure) and (ii) the system as also defined herein.

Especially, in some embodiments such device may further include a light source for generating radiation, such as one or more of UV and blue, which can be converted by In_xGai-_xP into light having a dominant wavelength in the range of orange to infrared, dependent upon x. Hence, in specific embodiments the device further comprises a light source configured to generate light source radiation wherein said In_xGai__xP is configured to convert at least part of the light source radiation into converter radiation. As indicated above, especially 0.25<x<0.7, even more especially 0.35<x<0.6. Hence, in such embodiments

In_xGai__xP is configured in a light receiving relationship with the light source. In_xGai__xP, such as the nano wires comprising In_xGai__xP, may in embodiments be comprised by a matrix, such as a light transmissive polymeric material. In a specific embodiment, the light source comprises a solid state light source (such as a LED or laser diode). The term "light source" may also relate to a plurality of light sources, such as 2-200 (solid state) LED light sources. Hence, the term LED may also refer to a plurality of LEDs. Further, the term "light source" may in embodiments also refer to a so-called chips-on-board (COB) light source. The term "COB" especially refers to LED chips in the form of a semiconductor chip that is neither encased nor connected but directly mounted onto a substrate, such as a PCB. Hence, a plurality of light semiconductor light source may be configured on the same substrate. In embodiments, a COB is a multi LED chip configured together as a single lighting module.

Alternatively, the device may be used in photolysis. Therefore, in embodiments the device is a photolysis device, and In_xGai__xP is comprised by an electrode configured for use in a photolysis process with said device. Hence, In_xGai__xP may also be used for direct solar light to hydrogen conversion (SLH), analogous to the use of InP in such applications, see e.g. Matthias M. May et al. in NATURE COMMUNICATIONS, 6:8286, DOI: 10.1038/ncomms9286, published 15 Sep 2015, titled "Efficient direct solar-to- hydrogen conversion by in situ interface transformation of a tandem structure^'", which is herein incorporated by reference. For SLH applications, the x- value may be less critical. Further, also a combination of a plurality of different core-shell nano wires may be applied, with different x- values. Further, the core-shell geometry may be especially useful for SLH applications.

The invention further relates to an electronic device comprising a first and a second electrode which are mutually connected through at least one nanostructure. The invention further relates to a method of manufacturing an electronic device, comprising: (a) providing growth nuclei of an electroconductive material on an electroconductive surface of a substrate, the surface being patterned so as to define a first electrode; (b) growing

nanostructures of a compound semiconductor material by chemical vapor deposition at a growth temperature; and (c) providing a second electrode that is in electrical contact with the nanostructures grown.

Hence, the invention provides in embodiments an electronic device

comprising a first and a second electrode that are mutually connected through at least one nanostructure that comprises a nanotube with a crystalline mantle and a hollow core. It is particularly preferred that the nanostructure comprises a first zone having a p-type doping and a second zone having an n-type doping, the first and second zones having a mutual interface constituting a pn-junction. The first zone may be one of the core (such as GaP) and a shell (such shell essentially consisting of In_xGai__xP), and the second zone may be of a shell (such shell essentially consisting of In_xGai__xP) and the core (such as GaP), respectively.

Herewith a light-emitting diode is obtained, in which the first electrode functions as a hole- injecting electrode and the second electrode as an electron- injecting electrode. Such light- emitting diode can for instance be used for display and lighting applications, as is known per se. However, the device with the nanotubes having a large quantum confinement and a suitable electroluminescent and photo luminescent effect is suitable as well for memory purposes (e.g. quantum dots), for ultrafast transistors and for optical switches, optocouplers and photodiodes (to convert an optical signal into an electrical signal or to do the reverse). As indicated above, especially 0.25<x<0.7, even more especially 0.35<x<0.6. In this way, the invention also provides a solid state lighting device, which may, upon application of a suitable potential difference to the electrodes, generate lighting device light due to the In_xGai_ _XP as defined herein.

The device of the invention can have various forms. It is advantageous if the nanostructures are present in an array within a layer, this layer separating the first and the second electrode. In this embodiment, the nanostructures are directed substantially transversal to electrodes. Advantages of this "vertical" type of device include that essentially no assembly of the nanostructures is necessary and that an array of nanostructures can be used for interconnecting both electrodes.

The layer in which the nanostructures are present can be provided before the growth of the nanostructures, e.g. as a porous matrix of for instance alumina. However, it can be provided afterwards as well, e.g. by growing the nanostructures and providing the layer from solution afterwards. A very suitable manner of providing such layer is sol-gel processing. A particularly advantageous layer comprises a mesoporous silica which may contain organic substitutions. Such a layer has a low dielectric constant, which reduces undesired capacitive interaction between the first and the second electrode. Alternatively, a polymer can be used that is transparent if optical properties of the nanostructures are to be exploited. This has the advantage that a flexible device can be obtained. An example of a suitable polymer is BCB (Benzocyclobutene).

The array type of device is particularly suitable in combination with a nanostructure including a p-n-junction. Such an array will result in a very high light output power density. If the array has a density of 10¹⁰ pores and hence nanostructures per cm², the power density can be in the range of 10²-10⁴ W/cm². Further, due to the crystallinity of the nanostructures, the efficiency of the light emitting diode is high, for instance about 60 %.

In the case where a bipolar transistor in the nanostructure is desired, this can be realized with the matrix containing internal conductors, or in that a conductive layer is provided between two sublayers after growing the nanostructures. Also the growing of the nanostructures may be done in steps such that after a first growth step, a first sublayer is provided. Then the conductor can be deposited, whereafter the growth process is continued in a second growth step, with the same metal nuclei. In order to improve the contact between such internal conductor and the nanostructure, it is preferred that the nanostructure has a mantle with a larger diameter or is a nanowire with a larger diameter at the contact with the internal conductor.

The layer may further be structured according to a desired pattern. This is particularly advantageous if the nanostructure is used as a photodiode. In that case the layer can be structured so as to have a fiber-like shape. Around the structured layer black or non- transparent layers can be provided, so as to keep the light inside the layer.

In further embodiments, the layer contains nanostructures of different materials. Herewith a multicolor light-emitting device is realized. The nanostructures of different materials can be provided in that a plurality of growth cycles is done, with first the provision of the nuclei, generally a droplet of a metal and then the growth at one or more desired growth temperatures, and then the removal of the nuclei, so as to stop the growth.

If used as a light-emitting diode, at least one of the electrodes is preferably transparent (such as Zno or ITO). At the side of the layer opposite to the transparent electrode, a reflecting layer may be present, so as to increase the efficiency of the light output.

Nonetheless, the advantages of the vertical device type, the nanostructures of the invention may be present in a thin- film device type, wherein the first and second electrodes are laterally spaced apart. A dispersion with the nanostructures, for instance in ethanol as dispergent, is then provided onto the electrodes. The alignment of the nanostructures and the electrical contacting between electrodes and nanostructures can be realized in a manner known per se, as is also disclosed by Lieber et al. (CM. Lieber and coworkers - Nature 2002, 415, 617-620 - and K. Hiruma and coworkers - Appl. Phys. Lett. 1992, 60, 745-747), which are herein incorporated by reference. Further information concerning the device embodiments can also be found in WO2004042830, which is herein also incorporated by reference.

The material WZ In_xGai__xP also appears to provide polarized light. As indicated above (and below), this may be used in all kind of applications. Hence, the lighting device may provide polarized light. Hence, in yet a further aspect the invention also provides a lighting system for providing (polarized) lighting system light, wherein the light system comprises the device as described herein, especially such device comprising a system with a regular arrangement of the nanowires. Therefore, in specific embodiments the lighting system as defined herein comprises the system as also defined herein. However, other options may also be possible. The lighting system may e.g. be used for providing headlight of a motorized vehicle. However, the lighting system may e.g. also be used as display backlight or in projection lighting, etc. etc.

In yet a further aspect, the invention also provides a dispersion of nanowires as defined herein in a solvent. In yet a further aspect, the invention also provides a polymeric material with nanowires as defined herein embedded in the polymeric material.

Hence, in specific embodiments the invention provides a device wherein said

In_xGai-_xP is configured to generate light. Such device may be configured as a light-emitting diode, i.e. with contacts on top and bottom of the nanowires. Even more especially, such device may be configured as a light-emitting diode, i.e. with contacts to the core and to an In_xGai__xP of the nanowires. As indicated above, the core may be doped with another type of doping than the In_xGai__xP. Between the doped core and doped shell, and intermediate In_xGai_ _XP shell may be available.

Thanks to their uniqueness, nanowires allow the exploration of novel semiconductor crystal structures with yet unexplored properties. Herein we show the investigations on the growth and optical properties of wurtzite In_xGai__xP as a function of the Indium concentration. Wurtzite In_xGai__xP is grown as an epitaxial shell around GaP wires, so that the wurtzite crystal structure of the core is transferred into the shell. The inventors report tunable light emission in the visible region between 590nm (2.1eV) and 760nm (1.63eV). The photo luminescence internal quantum efficiency of wurtzite In_xGai__xP steeply rises from 0.01% to 18% around x=0.4 due to a pseudo-direct (Tsc- r ) to direct (r_7c- r ) transition crossover. Our work reveals the electronic properties of wurtzite In_xGai__xP and explores its relevance for applications in solid-state lighting and solar light-to-hydrogen conversion.

Especially, the In_xGai__xP shell differs from the core in one or more of chemical composition and doping. Hence, the chemical composition may be different and/or the doping may be different. Further, at least the chemical composition is different, with the core e.g. comprising GaP or another Wurtzite material.

The lighting device may be part of or may be applied in e.g. office lighting systems, household application systems, shop lighting systems, home lighting systems, accent lighting systems, spot lighting systems, theater lighting systems, fiber-optics application systems, projection systems, self- lit display systems, pixelated display systems, segmented display systems, warning sign systems, medical lighting application systems, indicator sign systems, decorative lighting systems, portable systems, automotive

applications, (outdoor) road lighting systems, urban lighting systems, green house lighting systems, horticulture lighting, or LCD backlighting.

As indicated above, the lighting unit may be used as backlighting unit in an

LCD display device. Hence, the invention provides also a LCD display device comprising the lighting unit as defined herein, configured as backlighting unit. The invention also provides in a further aspect a liquid crystal display device comprising a back lighting unit, wherein the back lighting unit comprises one or more lighting devices as defined herein.

The term white light herein, is known to the person skilled in the art. It especially relates to light having a correlated color temperature (CCT) between about 2000 and 20000 K, especially 2700-20000 K, for general lighting especially in the range of about 2700 K and 6500 K, and for backlighting purposes especially in the range of about 7000 K and 20000 K, and especially within about 15 SDCM (standard deviation of color matching) from the BBL (black body locus), especially within about 10 SDCM from the BBL, even more especially within about 5 SDCM from the BBL.

The terms "violet light" or "violet emission" especially relates to light having a wavelength in the range of about 380-440 nm. The terms "blue light" or "blue emission" especially relates to light having a wavelength in the range of about 440-495 nm (including some violet and cyan hues). The terms "green light" or "green emission" especially relate to light having a wavelength in the range of about 495-570 nm. The terms "yellow light" or "yellow emission" especially relate to light having a wavelength in the range of about 570- 590 nm. The terms "orange light" or "orange emission" especially relate to light having a wavelength in the range of about 590-620 nm. The terms "red light" or "red emission" especially relate to light having a wavelength in the range of about 620-780 nm. The term "pink light" or "pink emission" refers to light having a blue and a red component. The terms "visible", "visible light" or "visible emission" refer to light having a wavelength in the range of about 380-780 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Fig.l (a) Schematics of the nanowires 200 in this work. Left: WZ GaP with the gold catalyst droplet G. Right: WZ GaP/InGaP core-shell nanowire (InGaP indicated with reference 1, notice the absence of the catalyst), (b-c) SEM images of two different core-shell samples. The bending due to asymmetric strain is visible in some nanowires. (d) Close-up SEM image of (c) to better show nanowire morphology and bending. Reference 300 indicates a system with nanowires 200 on a support 310, arranged in an 2D array 250. Reference d3 indicates a shortest distance between adjacent nanowires 200;

Fig. 2 (a) TEM image of a WZ GaP/Ino.23Gao.77P core shell nanowire, showing a missing crystal plane in the 15nm thick shell. Inset: Fast Fourier Transform of the shown image, showing the pattern of the wurtzite crystal structure (b) EDS compositional line profile of the same nanowire showing the achieved composition control, with some limited asymmetry in the shell. Inset: HAADF image to show the executed line scan. The length scale of the EDX line profile has been converted to a radial profile. A slight shell thickness dishomogeneity (<5nm) is also visible. On the y-axis the atomic percentage % is indicated, (c) 2D EDS mapping (left) and HAADF-STEM image (right) of a WZ GaP/Ino^Gao.vsP core shell nanowire in cross section, (d-e) Quantified EDS element maps of the same lamella for Indium (d) and Gallium (e). (f) FEM simulations of WZ GaP/Ino.25Gao.7sP (upper panel) and WZ GaP/Ino.75Gao.25P (lower panel) core-shell nanowires. The scalebar is common and the tensile strain is not displayed, as it is out of the scale;

Fig. 3 (a) Photoluminescence spectra at 10K of various WZ GaP/In_xGai__xP core shell nanowires, emitting in different spectral regions. Excitation power density:

3kW/cm². On the y-axis the photoluminescence intensity in arbitraty units is indicated; on the lower x-axis the wavelength in nm and on the upper x-axis the energy in eV; (b) Radial plot of intensity in function of signal polarization, superimposed on a panchromatic image of an emitting WZ GaP/In_xGai__xP nanowire. The perpendicular polarization confirms WZ emission (c) Lifetimes at 4K of two nanowires with different composition, showing an order of magnitude of difference in lifetime. On the y-axis the photoluminescence intensity in arbitrary units is indicated (log scale). Excitation power density: 0.2kW/cm² (d) Emission lifetime at 4K in function of Indium fraction: with x<0.30, the emission has a long lifetime, with x>0.35, the lifetime is one order of magnitude shorter. Excitation power density: 0.2 kW/cm². On the y-axis the lifetime in ns is depicted;

Fig. 4 (a) Integrated PL intensity, indicated with I (arbitrary units), in function of the inverse of temperature for pure WZ GaP and WZ GaP/Ino.₆Gao.4P core shell nanowires. The emission intensity at 300K is about 3 orders of magnitude different, (b) IQE in function of Indium fraction. All samples with x<0.30 showed very low IQE, while all samples with x>0.35 showed much higher IQE, up to 18%. (c) Measured emission energy (y-axis energy in eV) in function of Indium fraction. Dashed line starting left at about 2.2 eV: Γ8< -Γ9ν transition, solid starting left at about 2.7 eV: Γγο-Γξΐν transition. Γ₇ο-Γ9ν data was fitted parabolically. The Γ8ο-Γ_7ο crossover is estimated to lie around x=0.43. Dashed line starting at about 2.9 eV: simulated Γ₇ο-Γ9ν transition under 1% compressive hydrostatic strain, (d) Simulated data for the main transitions in WZ In_xGai__xP (y-axis energy in eV);

Fig. 5 schematically depicts some embodiments;

Fig. 6 schematically depicts a nanowire;

Fig. 7 schematically depicts some embodiments and variants of a lighting device and/or lighting system;

Fig. 8 schematically depicts a further embodiment of the device as described herein.

The schematic drawings are not necessarily on scale. DETAILED DESCRIPTION OF THE EMBODIMENTS

WZ GaP/In_xGai-_xP core-shell nanowires are grown by a multi-step method, featuring core VLS growth, ex-situ Au catalyst etching to suppress further axial growth and finally the growth of the InGaP shells (schematic layout in Fig. la). The Au catalyst droplets for VLS growth were fabricated using nanoimprint lithography on a square matrix with a 500nm pitch or on a hexagonal matrix with 2500nm pitch. The WZ GaP cores were then grown with Metalorganic Vapor Phase Epitaxy (MOVPE) at 615°C using Trimethylgallium (TMGa) and Phosphine (PF ) as precursors and HC1 as in-situ etchant to suppress sidewall tapering. The Au catalyst was subsequently removed from the nanowire top with an ex-situ wet etching, using King's Water and Iodine solution with optimized concentration and etching times (see below). TEM investigation showed complete elimination of Au from the nanowire, while leaving atomically smooth sidewalls and generating no defects. The etched cores were then used as a template for the shell growth at 585°C in the same reactor, using Trimethylindium (TMIn) as Indium precursor with a very high V/III ratio (>1000 in our case) to promote layer growth. In Fig. lb-d we can see that the wires may bend with an angle up to about 10°. In_xGai-_xP in fact possesses a larger lattice parameter than GaP and we expect the shell to be compressively strained: the bending is due to eventual asymmetrical defect density and/or composition in the shell and will be investigated more in detail in a future work. The optical studies performed in the present work have taken these factors into account using theoretical models.

Although the Au catalyst has been removed after core growth, during the growth of the WZ In_xGai__xP shell a small portion of ZB In_xGai__xP is grown on top of the wire, compatible with previous observations. In order to obtain high optical quality and unambiguous spectra of WZ In_xGai__xP, it is desirable to suppress the formation of such a ZB In_xGai__xP domain. The growth temperature and HC1 flow are therefore optimized to control the morphology of the layer growth. HC1 has two important effects which affect the shell growth: firstly, it etches material from the nanowire surfaces, preferentially etching Indium over Gallium. As the ZB domains are rich in Indium, they are more effectively etched by HC1. This effect therefore can also be used as an additional degree of freedom to control the effective Indium incorporation during the layer growth. Secondly, it passivates the surface of Ill-Phosphides, which leads in time to a saturation of the surface by Chlorine and stops the growth, limiting the maximum thickness of the shell to about 20-40 nm. This surface passivation effect can be eliminated by applying a growth recipe featuring two alternating steps: a 15 minutes long growth step as we described earlier, alternated by a 45s step to remove the passivation layer. The passivation removal step is performed with a lower flow of HC1 and a higher flow of Gallium, providing no Indium. During this step, TMGa reacts with Chlorine at the sidewalls, producing GaCb and thereby removing the passivation layer. Using this cyclic growth, an arbitrarily thick WZ InGaP shell can be grown.

In order to study the optical properties of WZ In_xGai__xP, we strive at ensuring a high crystalline quality of the shells, i.e. the purity of the WZ phase and the presence of defects, devoid of ZB segments that might cause spurious emission. To this end, the crystal quality of the core-shell nano wires is analyzed with Transmission Electron Microscopy (TEM). The overall structures need to be sufficiently transparent to the electron beam in order to allow high-resolution imaging, and therefore a maximum diameter of about 140 nm is used with a 15nm shell thickness. As shown in Fig. 2a, the In_xGai__xP shell shows a WZ crystal structure. Shells with low Indium content (up to about 15%) are defect-free, while the ones with higher Indium content may show edge dislocations, as in Fig. 2a. The defect density increases with In concentration due to increasing lattice mismatch between the GaP core and the In_xGai__xP shell. The shell composition is determined by Energy-dispersive X- Ray Spectroscopy (EDS), as shown in Figures 2b-d. The experiment is performed in two different crystallographic orientations: in projection (Fig. 2b) along the [1120] zone axis, and in cross-section (Fig. 2c-e), with the nanowire cut perpendicularly to its main [θθθΐ] axis.

Cross-sectional analysis is desirable to observe the surface faceting and to correctly evaluate the composition of a thick shell. The inventors used the EDS maps to calibrate the precursor flows to obtain the desired average composition of each shell. From the cross-sectional EDS analysis shown in Fig. 2c-e we obtain an average Indium composition of 25% with only minor compositional inhomogeneities of about 2%. Regions with lower Indium concentration are visible along the 'spokes' extending from the corners of the hexagonal core, a

phenomenon also observed in previous work on core-shell nano wires. A thin inner shell with slightly lower Indium concentration (~2%) can also be observed in fig. 2d, resulting from the growth during the Chlorine "de-passivation" step after growing the first In_xGai__xP shell. The facets of the WZ In_xGai__xP shells belong to two different families: {1100} and {1120}. More facets develop with increasing shell thickness, as it is commonly observed in many cases of radial growth on nanowires most likely because of surface energy minimization.

In order to evaluate the strain magnitude and distribution within the core/shell system, Finite Element Method (FEM) simulations of planar and hydrostatic strain have been performed for a WZ GaP/In_xGai__xP core-shell nanowire with a core apothem and shell thickness both of 50nm. The results are displayed in Fig. 2f for x=0.25 and x=0.75. A six- fold symmetry is clearly present, with six "spokes" at lower strain in the shell, as caused by the geometric relaxation induced by the six corners, in agreement with previous studies. This phenomenon results in a much lower average strain (less than one third) than expected for a corresponding planar {1100} heterojunction with identical lattice mismatch (Table 1): Table 1

The optical properties are investigated by photoluminescence (PL) measurements as a function of the In concentration. The inventors directly correlated the emitted PL wavelength with In composition by transferring nanowires onto a TEM grid and performed PL first and then EDS studies on the same nanowires. With this method we can correlate emission and composition without being affected by eventual sample

inhomogeneities. By varying the WZ In_xGai__xP composition (x=0.25-0.70), the emission wavelength can be tuned in the range from 590nm to 760nm as shown in Fig. 3a. The emission peaks have a Full Width at Half Maximum (FWHM) around 25nm. Such FWHM is comparable to WZ InP, indicating an overall good homogeneity of the emitting material, a result supported by the cross-sectional study previously illustrated. The emission of the WZ In_xGai-_xP shells is strongly (>85%) polarized perpendicular to the growth c-axis (shown in Fig 3b) demonstrating the WZ crystal structure. However, as both the Tsc and r₇c conduction bands of WZ In_xGai__xP are expected to emit with polarization perpendicular to the nanowire, the polarization selection rules do not allow to discriminate between these two bands.

Since the pseudo-direct (Tsc - Γ9ν) transition is allowed with a small oscillator strength and the direct (r_7c - Γ9ν) is expected to be allowed with a large oscillator strength, measurements of the radiative lifetime as a function of the Indium composition are expected to provide information about the Tsc and T₇c crossover. Two representative measurements are shown in Fig. 3c showing an order of magnitude of difference in the PL lifetime between x=0.25 and x=0.6 (WZ In_xGai__xP). Measurements performed over a wide compositional range at 4K show that the emission from WZ In_xGai__xP shells with x<0.25 feature lifetime between 1.9ns and 6.5ns, while shells with higher Indium composition feature a lifetime below 0.5ns, as reported in Fig. 3d. This relevant and reproducible change in lifetime provides a first signature of the Tsc and r₇c crossover resulting in a marked change of the electronic transition strength when passing from x=0.25 to x=0.38.

From temperature dependent PL measurements we can further study the Tsc - Tic cross over point. The inventors have studied the PL at temperatures between 4K and 300K, and we found a shift of the emission wavelength for all alloys consistent with

Varshni's model. In the WZ Ino.6Gao.4P sample shown in Fig 4a we see an increase of the integrated PL around 58K, corresponding to a thermal energy of 4meV, which we attribute to carrier detrapping from defects such as impurities and dislocations. The inventors determined the IQE of the samples by measuring the integrated PL intensity as a function of temperature, as shown in Fig. 4a. At low Indium fractions we obtain an IQE of about 0.01%, while beyond x=0.38 the IQE steeply rises by three orders of magnitude, reaching a maximum of 18%, as reported in figure 4b. Being Tsc - Tgv only a weakly allowed transition, in WZ GaP and Indium-poor WZ In_xGai__xP the emission is mostly impurity related. At high temperatures, carriers are de-trapped from the impurities by thermal energy and only the band-to-band transition is therefore possible, providing with the low IQE. In Indium-rich WZ In_xGai__xP however the lowest energy transition is expected to be the allowed T₇c - Tw, leading to a high IQE. We therefore again obtain strong evidence for the Tsc and T₇c crossover at an Indium fraction in the range 0.25<x<0.38. We note that at higher In concentrations the increased lattice mismatch results in a higher defect concentration and therefore in a lower IQE. Quantum efficiencies at 300K etc. are based on the assumption that the quantum efficiency at 10 K is 100%.

This increase in IQE in the range 0.25<x<0.38 is directly correlated with the shortening of the photo luminescence lifetime shown in Fig. 3. We can immediately exclude the influence of surface states on the lifetime shortening, as the IQE increase contradicts such hypothesis. All together, these data clearly demonstrate efficient direct recombination for WZ In_xGai__xP shells with an Indium fraction above 35%. By plotting the obtained emission wavelengths as a function of the WZ In_xGai__xP composition and indicating high (>1% in blue) and low (<0.01 > in red) IQE, we can find the crossover position of the two conduction bands as a function of the Indium fraction, shown in Fig. 4c. We complete the dataset with data from literature about the T₇c band position in WZ GaP and Tsc position in WZ InP. In the figure we can distinguish between Tsc - Γξ>ν related emissions (dashed line starting left at about 2.2 eV) with low IQE, longer lifetime and T₇c - Γξ>ν related emissions (triangles) with high IQE, short lifetime. We choose to fit linearly the data of the Tsc - Tgv transition, imposing the values for WZ GaP and WZ InP as constraints. We fit the T₇c - Γξ>ν data parabolically choosing again the energy position in WZ GaP and WZ InP as constraints, with a bowing parameter b = 0.8±0.1 eV. Finally, we can determine the crossover point around x=0.43 Indium which is in good agreement with our experimental data.

In order to validate our understanding of the optical properties, we also simulated the band structure of WZ In_xGai__xP at different compositions and hydrostatic strain conditions. The simulations have been performed using total-energy calculations in the framework of the density functional theory with an exchange-correlation functional in local density approximation as implemented in the Vienna ab initio simulation package (VASP). In Fig. 4d we show the obtained transition energies, fitted parabolically and presented in a similar fashion to Fig. 4c to facilitate the comparison. The results validate our attribution of the PL emission to the Tsc - Γξ>ν transition for Indium-poor WZ In_xGai__xP and to the r₇c - Γξ>ν transition for Indium-rich WZ In_xGai__xP. We also note that the M minimum cannot be responsible for the observed emission as it is definitely higher in energy to the peaks we observed. In Fig. 4c we also show (dotted or dashed line starting left at about 2.9 eV) the calculated effect of 1% compressive hydrostatic strain on the T₇c - Γξ>ν transition. In fact, although most of the compressive strain in WZ In_xGai__xP is relaxed by the dislocations in the shell, one cannot rule out that in a particular nanowire the strain might still be non-zero. This simulation result suggests that the spread we obtained in the datapoints might be due to compressive strain, which pushes the T₇c - Γξ>ν transition to higher energies.

The simulation yields a composition value for the Tsc - ic crossover of x=0.52, only in minor discrepancy with the value calculated from experimental data of x=0.43. This minor discrepancy is reasonable, considering the predictive power of the quasiparticle calculations as not better than ±0.1 eV, the assumed random distributions of the In and Ga atoms, and the assumed homogeneous strain. In Fig. 4c we also show (dotted or dashed line starting left at about 2.9 eV) the calculated effect on the T₇c - Γξ>ν transition of 1% compressive hydrostatic strain (calculated with FEM simulations (Table 1)). This aspect has to be taken into consideration as although most of the compressive strain in WZ In_xGai_ _XP is relaxed by the dislocations in the shell, one cannot rule out that in a particular nanowire the strain might still be non-zero. This simulation result suggests that the spread we obtained in the datapoints might be due to the predicted compressive strain, which pushes the T₇c - Γξ>ν transition to higher energies.

In summary, we demonstrated the growth of WZ GaP/In_xGai__xP core-shell nanowires in the interval 0<x<0.75, emitting in the visible range between 590nm (2.1eV) and 750nm (1.6eV). We observed the shortening of the emission lifetime by one order of magnitude between 0.25<x<0.40, coupled with a steep rise in IQE from 0.01% to 18% in the same composition range. We interpreted this steep change in lifetime and IQE to the change from T8c - Γξΐν to r₇c - Tgv type of emission. Our results open the way for high efficiency solar hydrogen conversion and LED devices based on WZ In_xGai__xP nanowires.

Below, some experimental aspects are discussed.

The Au catalyst on top of the WZ GaP nanowires is especially removed, as otherwise it may cause defective axial growth which will promote the formation of a large ZB grain on top of the wires.

A catalyst etching procedure that is used is:

- Rinse in a H₂0:H₃P0₄ 10: 1 solution for 30s

Rinse in Ultra Pure Water (UPW)

Etch in diluted (2: 1) king ' s water 45 s

Rinse in UPW

Etch in diluted KI/I2 solution ~15m

- Rinse in UPW

Rinse in Isopropanol (IP A)

Centrifuge

Times and concentrations can vary: higher concentration equals less etching time. A slow etching is preferable to prevent accidental overetching on the sidewalls.

Overetching will lead to incontro liable shell overgrowths. The final rinse in IP A and centrifuge will prevent nanowires to stick together during drying due to surface tension. If done correctly, the process ensures perfect epitaxy during shell growth without the need of annealing steps (Fig. 2).

A total of 12 facets have been observed for a 75nm thick shell. Variability in number of facets has been observed between different nanowires with the same shell thickness, but always with facets belonging to the same aforementioned families. We observe that the introduction of Indium may lead to a tendency to produce nanowires with a mix of hexagonal and triangular symmetry, as such geometry possesses a higher surface to volume ratio and therefore offers more strain relaxation. Also, the side facets of the nanowires are not all equivalent, but divided in two groups according to the polarity of the surface. It is likely that the growth rate depends on the polarity of the surface, as observed in other material systems.

Fig. 5 schematically depicts embodiments of a material 100 comprising In_xGai-_xP (wherein 0<x<l , and wherein In_xGai__xP has the Wurtzite crystal structure). Herein, In_xGai-_xP is also indicated with reference 1. In_xGai__xP can be provided as such, e.g. in the form of nanowires (I), but may also be embedded in a matrix 2, as schematically depicted at (II). Fig. 5 also schematically depicts an embodiment wherein the material 100 comprises a first material 1 10 comprising said In_xGai__xP and comprising a second material 120, wherein the second material 120 has the Wurtzite crystal structure, and wherein the first material 1 10 is attached to the second material 120 (lowest schematic picture in Fig. 5), such as especially a core-shell nanowire (see also Fig. 6).

Fig. 6 schematically depicts such core-shell nanowire 200, wherein the core- shell nanowire 200 comprise a core 220 of a second material 120 having the Wurtzite crystal structure, and a shell 210 comprising a first material 1 10 comprising said In_xGai__xP, with the core 220 having e.g. a diameter dl , in embodiments selected from the range of 10-200 nm, and with the shell 210 e.g. having a thickness d2 in embodiments selected from the range of 5-200 nm. The nanowire 200 may have a length 11, in embodiments selected from the range of at least 20 nm.

The invention also provides a device 400 comprising at least one or more of (i) the material 100 and (ii) the system 300 see also Fig. 7. Such device 400 may further comprise a light source 10 configured to generate light source radiation 1 1. In_xGai__xP is configured to convert at least part of the light source radiation 1 1 into converter radiation 101. Reference 401 indicates lighting device light. Further, the invention also provides a lighting system 1000 for providing polarized lighting system light 1001 , wherein the light system comprises the device 400. In Fig. 7, the lighting device and lighting system are essentially the same. Note that the device (or system) not necessarily includes a light source. In contrast, the InGaP may also be configured as semiconductor light source, see also Fig. 8.

Fig. 8 shows an embodiment of the present device 400,1000 comprising a solid state light source, such as a light-emitting diode, functionally connected to a source of electrical energy. The gold droplet may especially have been removed (e.g. with wet etching). The drawings represent Wurtzite (WZ) GaP / InGaP core-shell nanowires 200 grown on a Zincblende (ZB) GaP wafer, oriented to expose a (1 1 1)B surface. The substrate is indicated with reference 72, which can thus be GaP. Reference 71 indicates an electrode (first or second electrode) or back contact. The back contact is a metallic alloy, Ti/Au or Ti/Pt/Au. The nanowires 200 can be doped in the following way (chronologically, from bottom to top): a p-section doped with Zinc, an n-section doped with S, an n+ section doped with S (with a higher concentration). The n+ section is meant to render the top contact ohmic (avoiding the formation of a Schottky diode). However, other options may also be possible (see e.g. Fig. 8b). The nano wires are passivated with a passivation layer 73, such as a silicon oxide (SiOx) layer or a silicon nitride layer (SiNx). The nanowires 200 (with passivation layer 73) can (then) be embedded in a material 74, especially an electrically non-conductive material. An example may be a polymeric material, especially e.g. BCB (Benzocyclobutene). After the etching of part of the BCB (e.g. an O2/CHF3 reactive ion etching (RIE) to etch back the extra polymer, such as BCB, which provides access to the nanowire tips.), another electrode (second or first electrode), such as ZnO or ITO, can be grown on top via e.g. chemical vapor deposition (CVD) or atomic layer deposition (ALD). This electrode (layer) is indicated with reference 75. At least one of the electrodes may especially be transmissive for light, such as ZnO or ITO. Instead of doping with S also Se can be applied. In some experiments, Se was applied.

The substrate may especially include p-GaP. There may be an optional layer between the substrate 72, which may thus be ZB GaP and the electrode 71. This intermediate layer, indicted with reference 78, may especially include P⁺-GaP.

Further, in a variant, the InGaP shell includes a first undoped InGaP material, indicated with reference 110a, and a second doped InGaP material, indicated with reference 110b. The shell especially comprises p-Gap. All essentially have the WZ structure. When other materials are applied for one or more of the layers or core, essentially the same structure may be chosen. In a core-shell geometry the core will be p-doped (like the wafer), then one or more undoped shells to work as active layers, then an external n-doped (InGaP) contact layer. As indicated above, other supports than GaP may also be possible. Hence, this structure includes a radial pn-junction geometry. Further, note that the type of doping can be reversed as well (core n-doped, etc.).

The diode can be manufactured as follows. After provision of the first electrode 71 by sputtering of a layer of Ti, a matrix 3 of porous anodic alumina was provided. It had a thickness of 0.2 micrometer and a density of pores of 10¹⁰ pores/cm³. These pores, each with a diameter of 20 nm, were vertically aligned. The alumina matrix 3 can be manufactured as described in accordance with the method described in WO-A 98/48456.

Further information on how to build devices or parts thereof can also be found in Cui et al, Efficiency enhancement of InP Nanowire Solar Cells by Surface Cleaning, dx.doi.org/10.1021/nl4016182 | Nano Lett. 2013, 13, 4113-4117, which is herein

incorporated by reference.

The term "substantially" herein, such as in "substantially all light" or in "substantially consists", will be understood by the person skilled in the art. The term "substantially" may also include embodiments with "entirely", "completely", "all", etc. Hence, in embodiments the adjective substantially may also be removed. Where applicable, the term "substantially" may also relate to 90% or higher, such as 95% or higher, especially 99%) or higher, even more especially 99.5% or higher, including 100%). The term "comprise" includes also embodiments wherein the term "comprises" means "consists of. The term "and/or" especially relates to one or more of the items mentioned before and after "and/or". For instance, a phrase "item 1 and/or item 2" and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species".

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention further applies to a device comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

CLAIMS:

1. A material (100) comprising a core-shell nanowire (200), wherein the core- shell nano wire (200) comprise a core (220) of a second material (120) having the Wurtzite crystal structure, and a shell (210) comprising a first material (1 10) comprising In_xGai__xP, wherein 0.35<x<0.6.

2. The material (100) according to claim 1, wherein the core (220) has a diameter (dl) selected from the range of 10-200 nm, and wherein the shell (210) has a thickness (d2) selected from the range of 5-200 nm, and wherein the nano wires (200) have lengths (11) selected from the range of at least 20 nm.

3. The material (100) according to any one of the preceding claims, comprising a GaP core (220).

4. The material (100) according to any one of the preceding claims, wherein the shell (210) comprises a doping and wherein the core (220) comprises a doping, and wherein the shell (210) has another type of doping than the core (220).

5. The material (100) according to any one of the preceding claims, wherein the core-shell nanowire (200) comprises a radial pn-junction geometry.

6. A system (300) comprising a support (310) and a plurality of core-shell nanowires (200) according to any one of the preceding claims arranged on said support (310), wherein adjacent nanowires (200) have shortest distances (d3) selected from the range of 10- 50,000 nm, and wherein the nanowires (200) are arranged in a 2D array (250).

7. The system (300) according to claim 6, wherein the support (310) comprises a GaP support.

8. The system (300) according to any one of the preceding claims 6-7, wherein the core-shell nanowires (200) are embedded in a polymeric material.

9. A device (400) comprising at least one or more of (i) the material (100) according any one of claims 1-5 and (ii) the system (300) according to any one of the preceding claims 6-8

10. The device (400) according to claim 9, comprising a first electrode and a second electrode which are mutually connected via In_xGai__xP as defined in any one of claims 1-6.

11. The device (400) according to any one of the preceding claims 9-10, further comprising a light source (10) configured to generate light source radiation (11) wherein said In_xGai-_xP is configured to convert at least part of the light source radiation (11) into converter radiation (101).

12. The device (400) according to any one of the preceding claims 9-11, wherein the device (400) is a photolysis device, and wherein In_xGai__xP is comprised by an electrode configured for use in a photolysis process with said device (400).

13. A lighting system (1000) for providing lighting system light (1001), wherein the light system comprises the device (400) according to any one of the preceding claims 9- 12.

14. Use of the lighting system (1000) according to claim 13, for providing headlight of a motorized vehicle, as display backlight, or in projection lighting, in in street lighting, in swimming pool lighting, for water way lighting, in indoor arena lighting, in fashion lighting, in machine vision system lighting, for quality inspection lighting, or retail lighting.